Ultrasonic imaging systems are known for detecting or imaging the internal structures of liquid, solid, and semi-solid materials. In operation, such apparatus typically includes an ultrasonic transducer that generates a beam of acoustic signals, which is transmitted into the material of interest and is reflected by various gradients or other physical features of the material. The beam may be focused at various depths within the material and may also be scanned so that the reflected acoustic signals may be used to provide image data about various aspects of the material.
In a particular application of ultrasonic systems in the field of medicine, ultrasonic imaging systems are used to examine or monitor the anatomical features of a patient. For example, the reflected signals may be received, analyzed, and processed to produce an image display that is representative of blood flow, tissue, or the structure of internal organs, such as the heart.
In a phased array ultrasound imaging system, an ultrasound transducer includes an array of transducer elements. The system includes a multiple channel transmitter and a multiple channel receiver connected through a transmit/receive switch to the transducer. Each transmitter channel causes a selected transducer array element to transmit an ultrasound pulse into an object being imaged. The transmitted ultrasound energy is steered along a transmit scan line and is focused by applying appropriate delays to the pulses transmitted from each transducer array element, so that the transmitted energy adds constructively at a desired focal point to form a transmit beam. A part of the transmitted ultrasound energy is reflected back to the transducer array by various structures that are in the path of the transmitted ultrasound energy.
The reflected ultrasound energy from an object or structure arrives at the array elements at different times. The received signals are amplified and are delayed in separate receiver channels and then are summed in a receive beam former to form a receive beam. The delay for each channel is selected such that the receive beam is steered at a desired angle and is focused at a desired depth. The delays may be varied so as to focus the beam at progressively increasing depths along a receive scan line as the ultrasound energy is received.
Ultrasound energy is transmitted along multiple transmit scan lines in a desired scan pattern, such as a sector scan, and the received signals are processed to produce an image of the region of interest.
In order to obtain the highest quality image, both the transmit beam and the receive beam could be focused at each point in the area being imaged. However, the time required to obtain an image in this manner would be prohibitive. In most prior art systems, the transmit beam is typically focused at a single focal depth, and the receive beam is dynamically focused in the scan plane. For both transmit and receive beams, the elevation focus is typically established by means such as an acoustic lens mounted on the transducer. As a result, the transmit beam is out of focus at points displaced from the transmit focal point, and the receive beam is out of focus in the elevation plane, except at a fixed focal point. These factors cause those portions of the image that are displaced from the focal points to be degraded in quality.
More recently, arrays with variable-elevation focusing have been developed. A typical elevation-focused transducer in a medical ultrasound imaging system may include an array of 64 to 256 elements. Each transducer element is divided in three or more segments in the elevation plane. The segments of each transducer element can be activated via respective channels in signal processing circuitry for focusing in the scan plane and the elevation plane. A dynamic aperture may also be effected when different active apertures of the transducer are activated by selectively enabling different groups of transducer elements and segments. See, for example, U.S. Pat. No. 5,301,168, assigned to the assignee of the present application, which discloses an ultrasound transducer having rows and columns of transducer elements, and U.S. Pat. No. 5,462,057, assigned to the assignee of the present application, which discloses a phased array ultrasound transducer divided into transducer elements arranged side-by-side in the lateral direction.
However, as the array size increases, the demand for channels will increase as well. There are a limited number of beam forming channels, such as 256 or less, available in a typical ultrasound imaging system. For best use of these beam forming channels, it is known to divide each element into several elevation elements and to use symmetry to connect elements at equal distance from the center line of the transducer array. A typical array includes a distribution of transducer elements in what is known as a 1.5D linear array or a 1.5D curved linear array (CLA) transducer probe. Each transducer element is divided into one central element segment and two, four, or more smaller outer element segments. The plural segments may be arranged in an inner segment array, first outer segment array, second outer segment array, and so on. One may appreciate that each element of the array is thus divided into segments to allow electronic focusing in both the scan and elevation planes. The advantage of this approach is that the elevation focal point can be changed to allow the desired focus at a range of depths in the image. For example, if there are n channels devoted to each lateral element, then each lateral element can be divided into (n-1) elevation elements.
When operating a segmented array having an acoustic lens for elevation focusing, dynamic elevation focusing will provide only modest improvements in the far field, i.e., at points in a range beyond the focal point of the lens. However, great improvement is possible in the near field, i.e., at points in a range within the focal point of the lens, if the elevation dimension of the individual elements is small enough to prevent large phase errors across the width of the elements during focusing. Hence, segmented arrays having many small elements are attractive for use in many applications.
However, if the entire elevation aperture is filled with small elements, the demand for channels for signal processing cannot be met in a practical or cost-effective fashion. While features such as high resolution acoustic imaging, electronic beam steering, and electronic focusing provide many advantages, there remains a need for improvement in the design and construction of an ultrasonic probe suitable for effecting improved elevation focusing in the near-field.